1. Field of the Invention
The present invention relates to photovoltaic devices, such as solar batteries and sensors. In particular, the present invention relates to a photovoltaic device composed of nip-type silicon semiconductive layers including an i-type microcrystalline semiconductive layer, and a method for making the same.
2. Description of the Related Art
Photovoltaic devices or photoelectric transducers convert solar light energy into electrical energy, and have become widespread as low-power light sources in industrial fields including electronic calculators and wristwatches. Furthermore, photovoltaic devices have attracted attention as feasible electrical sources in place of chemical fuels, such as petroleum and coal.
A photovoltaic device uses photovoltaic conversion in a p-n junction in a semiconductor. Silicon semiconductors absorb solar light to form optical carriers including electrons and positive holes. The optical carriers drift due to the internal electric field at the p-n Junction and are supplied to an external device. Photovoltaic devices are generally produced by processes used in production of general semiconductor devices. A p- or n-type single-crystal silicon is formed by a crystal growth process, such as a Czochralski (CZ) process, and then is sliced to form silicon wafers with a thickness of approximately 300 .mu.m. A conductive layer is formed on each wafer, in which the wafer and the conductive layer have reverse types to each other, and thus a p-n junction is formed therebetween.
Single-crystal silicon has been primarily used in practical photovoltaic devices in view of reliability and conversion efficiency. Since these photovoltaic devices are produced by semiconductor production processes, their production cost is significantly high.
Single-crystal silicon photovoltaic devices have the following disadvantages. Single-crystal silicon has a low light absorption coefficient due to indirect transition; hence, it must have a thickness of at least 50 .mu.m in a photovoltaic device in order to achieve sufficient absorption of incident solar light. Single-crystal silicon has a band gap of approximately 1.1 eV, which is narrower than a desired value, 1.5 eV, for the photovoltaic device; hence, it is not capable of effectively using a short-wave region of the incident light.
A reported photovoltaic device using polycrystalline silicon has a surface texture structure, is capable of forming at a low temperature of 550.degree. C. or less, and has superior characteristics for a thickness of 5 .mu.m or less (Keiji Yamamoto, Akihiko Nakajima et al., "Optical Confinement Effect for below 5-.mu.m Thin Film Poly-Si Solar Cell on Glass Substrate", Jpn. J. Appl. Phys. Vol. 36 (1997), pp. L569-L572). This photovoltaic device, however, requires a relatively high process temperature and a low process speed, and has not resulted in practical use.
Since single-crystal silicon and polycrystalline silicon are composed of crystal, production of a large wafer is significantly difficult. Furthermore, photovoltaic devices used outdoors require expensive packages to be protected from mechanical damage caused by a variety of weather conditions. Thus, the production cost per unit of generated power is comparatively higher than that in conventional power generation methods.
Achievement of a low-cost, large-area photovoltaic device is an essential technical problem in practical use for generation of electrical power. A variety of studies, such as a search for inexpensive materials and materials with high conversion efficiency have continued for the solution to this problem. Examples of materials for photovoltaic devices include tetrahedral amorphous semiconductors, such as amorphous silicon, amorphous silicon-germanium, and amorphous silicon carbide; Group II-VI compound semiconductors, such as CdS and CU.sub.2 S; and Group II-V compound semiconductors, such as GaAs and GaAlAs. Among them, amorphous semiconductors are the most promising, since they have the following advantages. A thin-film photovoltaic device using an amorphous semiconductor as a photovoltaic layer allows production of a film having a large area. The thin-film photovoltaic device has a smaller thickness than that of single-crystal or polycrystalline silicon. The thin-film photovoltaic device can be deposited on any substrate material. On the other hand, amorphous semiconductor photovoltaic devices have disadvantages, such as low photovoltaic conversion efficiency and unreliability when they are used for the generation of large amounts of electrical power.
A means for the improvement of photovoltaic conversion efficiency of amorphous semiconductor photovoltaic devices is narrowing the band gap, which can result in increased sensitivity to long-wavelength light. Since amorphous silicon has a band gap of approximately 1.7 eV, it cannot absorb long-wavelength light with a wavelength of greater than 700 nm. Materials having a narrower band gap and thus having sensitivity to long-wavelength light are being studied. A typical material is amorphous silicon-germanium, in which the band gap can be readily varied in a range of approximately 1.3 eV to 1.7 eV by controlling the ratio of silicon material gas to germanium material gas in the film deposition process.
U.S. Pat. No. 2,949,498 discloses a stacked cell of a plurality of photovoltaic devices each having a unit device structure as another means for improving the cell's conversion efficiency. The stacked cell is composed of p-n junction semiconductors and has a technical conception common to amorphous and crystalline semiconductors. That is, individual photovoltaic devices have different band gaps and thus the stacked cell can absorb solar light with high efficiency, resulting in an increased photovoltaic conversion efficiency due to an increased V.sub.oc.
Stacked cells are designed so that the band gap decreases from the top layer lying at the side of incident solar light to the bottom layer lying at the side away from the incident solar light. The stacked cell can thereby sufficiently absorb all spectrum regions of the solar light, and thus the photovoltaic conversion efficiency is significantly improved (K. Miyauchi et al., Proc. 11th E. C. Photovoltaic Solar Energy Conf. Montreux, Switzerland, 88, 1992; and K. Momoto et al., "a-Si alloy Tree-Stacked solar Cells with High Stabilized-Efficiency II, 7th Photovoltaic Science and Engineering Conf. Nagoya, 275, 1993).
Photovoltaic devices including only i-type amorphous semiconductive layers cause a decrease in conversion efficiency with light irradiation, since amorphous silicon and amorphous silicon-germanium films deteriorate during light irradiation so that transportability of carriers decreases. Since such a phenomenon is not observed in crystalline semiconductors, the phenomenon is inherent in amorphous semiconductors. Such unreliability hinders practical use of amorphous semiconductors in the generation of electrical power.
Microcrystalline silicon semiconductors with i-type layers produced by a plasma enhanced CVD process have attracted attention, in view of the thickness for achieving a sufficient optical current and photo-deterioration (A. Shah, H. Keppner, et al., "INTRINSIC MICROCRYSTALLINE SILICON (.mu.c-Si:H)--A PROMISING NEW THIN FILM SOLAR CELL MATERIAL", 1994, IEEE First WCPEC, pp. 409-412, Dec. 5-9, 1994, Hawaii; A. Shah, H. Keppner, et al, The "Micromorph" Solar cell: Extending a-Si:H Technology Towards Thin Film Crystalline Silicon, 25th IEEE PV Specialists Conference, Washington, May 13-17, 1996). In these technologies, however, a slow deposition rate of the microcrystalline layer hinders practical use of such semiconductors.